ABCC6 Protein Research Articles

The human pseudoxanthoma elasticum gene ABCC6 is transcriptionally regulated by PLAG family transcription factors

Mutations in the ABCC6 gene are known as causative factors of pseudoxanthoma elasticum (PXE), a connective tissue calcification disorder, but the molecular mechanism of pathogenesis or the physiological function of ABCC6 protein is the subject of intense debate. The ABCC6 gene expression is tightly regulated at the transcriptional level and its tissue-specific distribution is consistent with PXE being a metabolic disease caused by failure of ABCC6 function in organs distant from the diseased sites. In an effort to provide clues to its role by elucidating the mechanisms of its regulation, we identified ABCC6 as a target gene for transcriptional induction by PLAG1 and PLAGL1, transcription factors from the PLAG family of cell cycle progression-related DNA-binding proteins. Both these factors are shown to bind to the same single consensus-binding element in the ABCC6 proximal promoter in cell lines of hepatic and renal origin by reporter gene assay, electrophoretic mobility shift assay and chromatin immunoprecipitation. PLAG-mediated ABCC6 transactivation may play an important role in determining the level of tissue-specific expression of this gene. The described mechanism can also find potential application in therapeutic interventions in forms of PXE related to impaired ABCC6 expression.

DNA methylation and Sp1 binding determine the tissue-specific transcriptional activity of the mouse Abcc6 promoter

The gene encoding the ABCC6 protein, an ABC transporter of the multidrug resistance-associated protein (MRP), is mainly expressed in liver and kidney. Mutations in ABCC6 are responsible for the development of the pseudoxanthoma elasticum (PXE) phenotype. PXE is a recessive disease characterized by the calcification of elastic fibers resulting in dermal, vascular, and ocular clinical manifestations. The physiological function of ABCC6 and the rodent orthologs Abcc6 is unknown and their precise relationship to elastic fibers is only a matter of speculation. Despite several studies focused on the transcriptional regulation of ABCC6/Abcc6, the molecular signals conferring the tissue-specificity to the ABCC6/Abcc6 expression are not well defined. In this report, we determined the level of the mouse Abcc6 promoter methylation in tissues with low level of expression (tail extremity and skin), intermediate (kidney), and high level of expression (liver). We observed that high and moderate levels of methylation correlated with low levels of Abcc6 expression. Moreover, we determined that CpG methylation of the Abcc6 proximal promoter region was interfering with the binding of the Sp1 transcription factor thereby inhibiting Sp1-dependent transactivation. Thus, our data provide the first direct evidence that an epigenetic mechanism regulates the binding of transcription factor Sp1 to the proximal promoter and participates in the tissue-specific expression control of the mouse Abcc6 gene.

Pseudoxanthoma Elasticum: the End of the Autosomal Dominant Segregation Myth

Pseudoxanthoma elasticum (PXE) is a heritable connective-tissue disorder affecting the eye, skin, and vascular system. Recent publications show that PXE exclusively segregates in an autosomal recessive fashion. However, the lack of an internationally accepted clinical "gold standard" for PXE, our incomplete knowledge of PXE etiology, and the incomplete nature of some molecular, clinical, and environmental studies warrant further investigation.

Analysis of ABCC6 (MRP6) in normal human tissues

To determine the tissue distribution of the ABC transporter ABCC6 in normal human tissues, we analyzed tissue arrays for the presence of ABCC6 mRNA by in situ hybridization and ABCC6 protein by immunohistochemistry using the polyclonal antibody HB-6. We detected ABCC6 mRNA and protein in various epithelial cells of exocrine and endocrine tissues, such as acinar cells in the pancreas, mucosal cells of the intestine and follicular epithelial cells of the thyroid. We obtained a very strong immunostaining for enteroendocrine G cells in the stomach. In addition, ABCC6 mRNA and protein were present in most neurons of the brain, in alveolar macrophages in the lungs and lymphocytes in the lymph node. Immunohistochemisty using the monoclonal antibody M6II-31 confirmed the widespread tissue distribution of ABCC6. The physiological substrate(s) of ABCC6 are yet unknown, but we suggest that ABCC6 fulfills multiple functions in different tissues. The strong immunostaining for ABCC6 in G cells suggests that it plays an important role in these endocrine cells.

Pseudoxanthoma elasticum: a clinical, histopathological, and molecular update

Pseudoxanthoma elasticum is an autosomally inherited disorder that is associated with the accumulation of mineralized and fragmented elastic fibers in the skin, Bruch's membrane in the retina, and vessel walls. The ophthalmic and dermatologic expression of pseudoxanthoma elasticum and its vascular complications are heterogeneous, with considerable variation in phenotype, progression, and mode of inheritance. Using linkage analysis and mutation detection techniques, mutations in the ABCC6 gene were recently implicated in the etiology of pseudoxanthoma elasticum. ABCC6 encodes the sixth member of the ATP-binding cassette transporter and multidrug resistance protein family (MRP6). In humans, this transmembrane protein is highly expressed in the liver and kidney. Lower expression was found in tissues affected by pseudoxanthoma elasticum, including skin, retina, and vessel walls. So far, the substrates transported by the ABCC6 protein and its physiological role in the etiology of pseudoxanthoma elasticum are not known. A functional transport study of rat MRP6 suggests that small peptides such as the endothelin receptor antagonist BQ123 are transported by MRP6. Similar molecules transported by ABCC6 in humans may be essential for extracellular matrix deposition or turnover of connective tissue at specific sites in the body. One of these sites is Bruch's membrane. This review is an update on etiology of pseudoxanthoma elasticum, including its clinical and genetic features, pathogenesis, and biomolecular basis.